This invention relates to a system for raising fish under a controlled environment, and more specifically for fish farming for commercial purposes at high density.
Fish husbandry under a controlled environment, sometimes referred to as fish farming, offers numerous advantages which are by now well known. Some of these advantages are the ability to provide large quantities of good quality high protein food at a relatively low cost, the opportunity to regularize the supply of gourmet fish subject in nature to seasonal fluctuations and the ability to conduct operations at inland locations thereby increasing the availability of fish to residents located far from the coastline.
Such commercial potential has stimulated interest in fish husbandry and it has been discovered experimentally that confined fish can be grown in much higher concentrations of fish per cubic foot of water than was formerly believed possible. For example under suitable conditions fish can be raised at concentrations in excess of 6 pounds of fish per cubic foot of water. To grow fish at this density commercially would be very advantageous as it would offer the opportunity to obtain a maximized yield of fish within the limitations of space available, and oxygen and water supply.
Currently known fish farming methods, however, would not generally be adaptable to fish farming at such high concentrations by reason of limitations inherent in their design which render it impossible to achieve the unusually high through-put of oxygen, food and water necessary to sustain fish life at such concentrations. For example one prior system utilizes a plurality of tanks arranged in a cascade with walls dividing each tank internally into a fish room and a filter room. Water passes through a filter in the bottom of the fish room, into the filter room from which the filter water overflows and falls into the open top of the fish room of the next succeeding downstream tank. Aeration of the water is through exposure of the surface of the water in the fish room to the atmosphere. Although satisfactory for its intended purpose, such a prior structure, relying solely on exposure of the water surface to the atmosphere, would not provide sufficient oxygen necessary to sustain the life of fish contained at extremely high concentration within the tank. This is because fish, at a high concentration, require very large amounts of oxygen to be supplied to the water (a) because of the aggregate breathing demands of the very large number of fish present, and (b) to oxidize and thus offset the toxic effects of the considerable output of waste products which the numerous fish produce in the body of water within which they are confined.
Additionally the prior structure described relying on a flow of water downwardly within each fish room does not provide any hydrostatic head of water assisting the oxygen into solution at the point at which it is introduced, namely the surface of the water in the fish room. Even if additional air were bubbled into the bottom of the water in the filter room so that the water was oxygenated prior to entering the next fish room downstream, the water would still enter at the water surface in the next fish room and much of the oxygen would pass out of solution there and be lost.
Another problem with the prior structure, if it were to be used for high concentration fish farming, would arise in connection with the high output of solid, fish waste products from the large number of fish involved, which would be carried downwardly by the flow of water in the fish room and accumulate on the bottom. To avoid a buildup of toxic products to a dangerous level and to prevent clogging of the water flow, it would frequently be necessary to drain the tanks for cleaning presenting a major problem in the fish raising operation.